Consider using `develop` branch

[dyladan]

Currently, we are merging all PRs into main and cutting releases periodically. I propose we change that flow so that all PRs go into a develop branch which is merged into main only on release. This has a few advantages:

main branch will reflect currently released state. To me this is the biggest advantage. In the current state, any new features documented in README files, examples, or other places are actually not useable by the latest release. Ensuring main always reflects the latest release will alleviate this issue.

Bug fixes can be easily released without new features that aren't ready. Currently, we have to release whatever is in main in order to get bug fixes out. This hasn't been a problem in the past, but as the project is maturing I think we will need to solve this issue. There are other ways to solve it, but this seems the simplest to me.

Release automations can run when a PR is merged into main. This is a secondary concern as there are other ways to solve it, but releasing when a branch is merged is a very simple workflow.

/cc @open-telemetry/javascript-maintainers